View Full Version : Ground Loop behavior: Teensy Audio board vs Adafruit AudioFx

07-12-2016, 05:34 AM
I'm hoping that somebody knowledgeable may help to shed some insight into a situation I've recently encountered.

I have a Teensy development setup in an environment with known (intentional) ground loops that are used for testing.

A laptop PC + audio interface + USB hub are fed from one AC circuit, and a mixer is fed from a different AC circuit entirely. By inserting audio gear between the PC and the mixer, I can reliably elicit ground loop noise in audio gear that is susceptible. Switching in an EBTech isolation transformer pair before the mixer reliably abolishes ground loop noise and permits A-B comparison.

Now I have a project that was originally prototyped using Teensy 3.2 connected to an Adafruit AudioFx board. In that case, ground loop noise does not appear so long as the analog and digital grounds are kept separate.

When I substitute the Teensy audio board, ground loop noise is back at unusually high levels. This is not AC hum, it's several different patterns of "digital" noise that appear on the headphone output only when a ground loop is present. Switching in the isolation transformers instantly cures the problem, but of course the users of the intended device will not have isolation transformers. Many of them will not have ground loop problems, either, but for those who do, the device as it stands would be unusable.

There is no particular dependency on software: the noise is present on the headphone jack at high levels in every example from the Audio Workshop.

In my first pass I have simply stacked all pins between the teensy and the teensy audio board. Now I wonder whether anybody has found a way of connecting the two that does not result in such high propensity to ground loop noise. I can un-stack the boards and begin playing with connections, but if anybody has specific suggestions or insight I would greatly appreciate the guidance.


-- Craig

10-10-2020, 11:38 PM
Hi Craig, I am currently working on building an acoustic monitor with a teensy 4.1 and Audio shield Rev D for a project. I am in a pickle, because this is also problem I am facing too where I suspect I have a ground loop problem... I am posting a reply directly here because I feel like all the solutions I have found so far to reduce noise while recording is not reliable and do not address this as a ground loop problem.. I am very new to this problem as well, which is giving me a lot of pains;)

I am using an electret microphone connected to GND and MIC on the audio shield and testing the device through recording normal sounds from my room and then storing it to an SD card(everything being powered from USB and testing using serial for now). The teensy is also stacked on top of the audio board, and all the connections are good. Upon seeing the spectogram, I am receiving a lot of background noise as well and I've suspected it to be a ground loop problem. Here is my setup and 2 spectograms with MIC BIAS voltage set at 1.25V and 2.5V (the background noise sounds more intense when I change this voltage, with 3V default resulting in the worst sound). The bottom spectogram shows me talking a bit then I remain silent.

22044 22045

I would like your advice on this... There is the Audio Ground Loop Noise Isolator on the teensy page, which is unavailable for me in my country (I cannot find the exact one). I googled a bit about this problem and read up more about ground loop noise a bit, and saw that 1:1 transformers can also work. Could you educate me more regarding this problem in terms of answering the following if you have answers for this by now?

1) After all these years, do you know what is happening with the question you've asked?
2) For my specific setup, where would you connect the ground loop noise isolator?
3) What are similar products to what you've used in this example and the Ground Loop Noise Isolator on the pjrc website??

I have also considered this to be a USB problem, so I will test with external power next week (only suspect this from reading a few forum posts).

Please let me know what you think man, I will really appreciate it!

Kind regards

10-10-2020, 11:48 PM
I use a USB ground isolator. Many available, usually based on an Analog Devices ADUM4160 chip or one of its family. I also give each project a good solid ground plane of sheet copper.

10-12-2020, 07:29 PM
I eventually tested it with external power using a 3x1.5V battery supply... I'm still getting the bad audio with no USB connection...

You can have a listen here https://www.dropbox.com/s/pm6ue9fl85wsf44/REC01.WAV?dl=0